Google on Tuesday launched online tools to help advertisers choose the best Web sites for reaching a target audience, a move that apparently sent the stock of Web metrics firm ComScore plummeting more than 22%.

The Google Ad Planner enables advertisers to enter the demographics and sites associated with their target audience to get back information about sites, on and off the Google network, that the audience is likely to visit.

The tool also lets advertisers drill down further to get more details, such as demographics and related searches for a particular site, Google said. In addition, an advertiser can get aggregate statistics for the sites an advertiser has added to a media plan.

Google unveiled Ad Planner several days after launching Google Trends for Websites, a tool that lets people view the popularity of Web sites based on numbers of visitors. The tool also compares and ranks site visitation across geographies and related Web sites and searches.

While Trends for Websites is for a general audience, Ad Planner is designed specifically for media planners within advertising agencies. The tool lets advertisers create media plans and then export them as a .CSV file to spreadsheet applications.

Online advertising experts said that while Ad Planner, which is free, might be useful to small advertisers and businesses, it's unlikely to steer larger advertisers away from ComScore and Nielsen Online, which are paid services for Web data. Advertisers would unlikely be willing to make spending decisions based on information provided by the Web's largest ad publisher, experts said.

"What's wrong with this picture," David Card, analyst for Jupiter Research, said in his blog. "The largest seller of online advertising apparently wants to be the go-to source for measurement data whose primary purpose is to sell advertising. Great job if you can get it."

Nevertheless, ComScore stock took a big hit following Google's announcement, dropping more than 22% to finish the day at $21.45.